The need for wavelength division multiplexers (WDMs) in today's laser-based transmission systems is well known. The ability to use multiple, different wavelengths to carry data signals, and then multiplex these separate data signals onto a single optical waveguide is useful.
Multiplexer arrangements have been developed that utilize an arrayed waveguide grating (AWG) device. An AWG is a planar structure comprising an array of waveguides that are positioned in a side-by-side configuration, with the array disposed between input and output couplers. These components then act together as a diffraction grating in a spectrometer. Each of the waveguides differs in length with respect to its nearest neighbor by a predetermined amount. In operation as a multiplexer, a plurality of separate and distinct wavelengths is applied to separate and distinct input ports of the device. These wavelengths are combined as they pass through the structure, exiting at a single output port. The signals need to be presented at the input in a monotonic sequence (either increasing or decreasing in wavelength value) so that all of the signals will appear at the single output port. The same device may be used as a demultiplexer, with a single waveguide supporting a plurality of signals operating at different wavelengths introduced to the “single port” of the device (the outputs then appearing at the plurality of separate and distinct ports at the other device termination, again the output signals appearing in monotonic wavelength sequence).
In situations where a monolithic array of laser diodes (each diode operating at a different wavelength) is used to provide a multiplexer input to the AWG, this monotonic wavelength sequence limitation necessitates that each separate laser diode forming the array be operating at its defined wavelength value within a given tolerance level (e.g., ±2-3 nm, at most). If the fabrication process results in only one wavelength value falling outside of this tolerance, the entire monolithic array must be discarded, incurring a significant expense.
Further, the desire to provide hybrid integration of laser arrays with optical multiplexers for packaged WDM components has presented difficulties in terms of the yield of acceptable arrangements. In particular, wavelength registration of the laser array and the optical multiplexer to the WDM grid as set by industry standards is critical (e.g., the ITU grid standard, which is an inter-channel spacing of 100 GHz). Wavelength registration is determined by fabrication tolerances of both the laser and the multiplexer. However, the fabrication capability is not sufficiently advanced to guarantee perfect wavelength registration to the ITU grid. While this is a problem for both the lasers and the multiplexers, the ability to provide laser diodes with the specific inter-channel spacing is more difficult. Past attempts at improving yield of the hybrid integration of laser arrays and optical multiplexers have involved activities such as “binning” the as-fabricated laser arrays and/or using temperature tuning of the individual laser diodes forming the array to modify the center wavelength values of each laser source. However, temperature tuning is limited by device performance and power consumption. Moreover, as the array ages, it may not be able to use temperature tuning to maintain an operational set of wavelength values.
These and other complications and drawbacks have thus limited the abilities to integrate laser arrays and multiplexers in a hybrid structure, as preferred for advanced packaging arrangements.